Magnetoresistive random access memory (MRAM) is a memory technology that often uses a magnetic tunnel junction (MTJ) to store information. The basic MTJ comprises a pinned layer, an intervening tunnel barrier and a free layer. Typically, a pinned layer comprises one or more pinned ferromagnetic sublayers deposited on an antiferromagnetic layer. The interaction of the pinned layer and the antiferromagnetic layer acts to establish stable magnetic orientations in the pinned ferromagnetic sublayers for applied magnetic fields up to several hundred or thousand oersteds. A free layer comprises either a simple thin layer of ferromagnetic material or a multilayer structure comprising ferromagnetic and spacer sublayers. The tunnel barrier is often formed by oxidation of a thin metal sublayer (usually Al or Mg) which is grown between the pinned and free layers or by deposition of an oxide or nitride layer. In addition, a basic MTJ may also comprise one or more cap layers and growth layers. A cap layer may serve several purposes. It may be used to improve the magnetic properties of the free layer, act as a thermal diffusion barrier and/or allow for improved adhesion to the MTJ device. A growth layer, on the other hand, is usually found near the bottom of the MTJ and is designed to promote high-quality growth of subsequently deposited layers.
The resistance of the MTJ is either high or low, depending on the relative polarization (parallel or antiparallel) of the free layer with respect to the pinned layer. If the magnetic polarizations of the layers are parallel relative to one another, the resistance across the tunnel barrier is usually lower, while if the magnetic polarizations of the layers are antiparallel, the resistance is usually higher. In order to switch an MTJ device (i.e., write to the memory cell), a particular sequence of magnetic field pulses is applied to the MTJ using on-chip current pulses through wordlines and bitlines placed near the device. The particular sequence of magnetic field pulses depends on whether the free layer consists of one or more than one ferromagnetic sublayer. Writing to MTJs comprising a free layer having a single ferromagnetic sublayer (which is typically switched using the Stoner-Wohlfarth method) is described in, for example, D. Worledge, “Single-Domain Model for Toggle MRAM,” IBM Journal of Research and Development, Vol. 50, No. 1, January 2006, p. 69, which is incorporated herein by reference. Writing to an MTJ comprising a free layer having two ferromagnetic sublayers (often called toggle or rotational switching) is described in, for example, the last cited journal article as well as U.S. Pat. No. 6,545,906, entitled “Method of Writing to Scalable Magnetoresistance Random Access Memory Element,” which is also incorporated herein by reference.
Ferromagnetic sublayers deposited in the presence of an external magnetic field are characterized by a magnetic anisotropy axis determined by the direction and magnitude of the external magnetic field. That is, a sublayer displays an unambiguous difference in behavior when subsequently exposed to an external magnetic field applied parallel and perpendicular to a certain magnetic anisotropy axis in the sublayer. The sublayer's magnetic moment vector points along its magnetic anisotropy axis in zero applied magnetic field. Moreover, the sublayer is more easily switched between magnetic polarizations when an external magnetic field is applied to the sublayer in the direction parallel to its magnetic anisotropy axis. One of the keys to obtaining high quality switching in MTJs is, therefore, precise control of this magnetic anisotropy.
Notably, it has been recently observed by the inventors that growth layers in an MTJ may affect the resultant dispersion in the direction of the magnetic anisotropy in subsequently deposited ferromagnetic layers. In many cases, this effect is substantial. There is, as a result, a need for a toggle-type MTJ having a structure allowing the magnetic anisotropy axes in the ferromagnetic layers to be precisely controlled. In this way, the magnetic switching characteristics of the MTJ can be optimized